Once Vista sets the activated flag, does it actually check for revocation of activation at some prescribed interval?

Why, yes. Rechecking the activation key against an updated list of revoked licenses takes place as part of the periodic updates to "Windows Validation" delivered via Windows Update. In practice under XP, this happens every month to every few months. Depending on your settings and whatever the future might bring, it might well be the case that machines will be checking for updates & possibly re-validating themselves every week.